In certain instruments, such as gyroscopes, there is a sensitive movable element (e.g., the gyro rotor) capable of small motions about its central position within the instrument housing. In these instruments, it is necessary to apply a force or torque to the sensitive element to force the element back to its central position. The manner in which the sensitive element is forced back to its central position is usually accomplished by applying a voltage to electrode plate pairs. The electrode plate pairs are separated by a narrow gap from the sensitive element along the X and Y axes. The plate electrodes parallel to the X and Y axes and the sensitive element form pairs of capacitors along these axes. When the sensitive element moves from its central position, a voltage is induced across the load resistor of the capacitive circuit in either the X or Y axis or both. This output signal is used by appropriate circuitry to cause the element to be forced back to its central position. Thus, the same X and Y axis capacitive circuits may be used as a pickoff or as a torquer.
A disadvantage of the instrument described is that there are independent X and Y axis pickoffs, each having its own scale factor determined by the electrode plate area and the nominal gap between the plate and the sensitive element. Another disadvantage is that there are instabilities in geometry; that is, if the electrode structure goes out of round, the average gap on the X-axis will differ from the Y-axis again changing the relative scale factors. Thus, the device will be unable to meet the scale factor requirement that the two pickoffs must be accurate to five parts per million in order to measure a displacement angle within an accuracy of one second of arc.